1. Field of the Invention
The present invention relates generally to light source devices used for areal illumination, and more particularly, to a light source which comprises a lamp having a bar-shaped luminous body and a light transmitting plate member for diffusing light from the lamp to function as a flat light source.
2. Description of the Prior Art
There has been proposed an image display apparatus operative to make images on a liquid crystal display panel. In such an image display apparatus, it is usual to put a light source at the back of the liquid crystal display panel for illuminating the liquid crystal display panel from behind so that the images displayed on the liquid crystal display panel is improved in contrast. The light source provided at the back of the liquid crystal display panel is formed into a flat light source for making areal illumination so as to apply uniform light over the whole rear surface of the liquid crystal display panel.
The flat light source for illuminating the liquid crystal display panel from behind is required to be relatively high in brightness without luminous nonuniformity. Therefore, there has been also proposed a light source in which light from a fluorescent lamp is diffused for functioning as a flat light source. A light source device proposed previously for functioning as a flat light source is constituted by way of example as shown in FIGS. 1 and 2.
In the light source device shown in FIGS. 1 and 2, a fluorescent lamp 11 having a bar-shaped luminous body is put in a lamp housing 12 having an opening 12A. The lamp housing 12 is provided with lamp supporting portions 13A and 13B for supporting the fluorescent lamp 11 at both ends thereof and making an electrical connection with the fluorescent lamp 11, and a reflective curved interior surface 14 for reflecting the light from the fluorescent lamp 11 toward the opening 12A.
A translucent plate member 15 is attached to the lamp housing 12 to be opposite to the reflective curved interior surface 14 with the fluorescent lamp 11 between for covering the opening 12A. The translucent plate member 15 is composed of a luminous nonuniformity reducing sheet 16 made of transparent polycarbonate film on which aluminium is put by means of vacuum evaporation to form a thin layer having a predetermined pattern and a diffusing plate 17 made of transparent acrylic resin plate containing diffusing agent dispersed therein, both of which are superposed on and incorporated with each other with adhesive tapes 18 and 19 stuck to their end faces, and bonded to front end faces of the lamp housing 12 surrounding the opening 12A.
There has been further proposed another light source device in which a diffusing plate, which is provided for diffusing light emitted from a lamp to pass through the diffusing plate directly or after reflected by a reflective surface positioned to face to the lamp, is constituted to have a Fresnel lens at a portion thereof through which the light from the lamp passes directly, as disclosed in, for example, Japanese patent application published before examination under publication number 63-101921.
In the light source device proposed previously as shown in FIGS. 1 and 2, the light from the fluorescent lamp 11 enters into the trasnlucent plate member 15 directly or after reflected by the reflective curved interior surface 14 and is diffused by the trasnlucent plate member 15 to pass through the same to the outside. Accordingly, the translucent plate member 15 acts as a flat light source to emit the light over the whole area. However, the light source device as shown in FIGS. 1 and 2 encounters with disadvantages that the light from the fluorescent lamp 11 is not effectively utilized so that it is difficult to have satisfactorily high brightness with power consumption restricted to be reasonable and power consumption is considerably increased when satisfactorily high brightness is required.
Further, in the case of the light source device which has the diffusing plate provided with the Fresnel lens formed at the portion thereof through which the light from the lamp passes directly, there is a problem that the light passing through the Fresnel lens formed on the diffusing plate is unsatisfactorily diffused and therefore a flat light source having sufficiently uniform brightness over the whole area can not be obtained.